A conventional boiling water reactor includes a reactor pressure vessel within which is disposed a nuclear reactor core having a plurality of fuel bundles. The core is effective for generating heat released from nuclear reactions for boiling water contained in the vessel for generating steam to power a steam turbine for driving, for example, an electrical generator for providing power to a utility grid. The reactor core typically includes a plurality of control rods or blades containing solid nuclear poison which are selectively inserted and withdrawn therefrom by conventional control rod drives (CRDs) for controlling the nuclear reaction rate, or reactivity, within the core. A typical nuclear reactor includes a substantial number of control rods and corresponding control rod drives, for example, over 200 of each. The control rod drives are typically mounted externally of the vessel at either the upper or lower closure head of the vessel and have push rods which extend through the vessel and into the reactor core. Conventional control rod drives either function hydraulically or electro-mechanically for positioning the control rods within the reactor core.
In both the top and bottom mounted control rod drives described above, relatively large apertures through the reactor pressure vessel are required by the drives for translating the control rods, and, therefore, require suitable sealing to prevent leakage of the high pressure water contained in the vessel. Furthermore, bottom mounted CRDs are disposed below the reactor core which complicates maintenance procedures for preventing leakage or draining of the water from the vessel when individual CRDs are removed during maintenance.
Bottom mounted CRDs also require increased servicing area below the vessel which necessarily requires a taller containment building, and attendant increased cost. Since the number of conventional CRDs typically is greater than about 200, the area under the vessel is relatively congested, and requires suitable access areas for required maintenance operations. Conventional recirculation pumps also typically extend from the bottom of the vessel adjacent to the CRDs, which additionally congest the area under the vessel.
On the other hand, top mounted CRDs present the additional problem that all of the service lines to the CRDs must be disconnected and removed prior to removal of the CRDs, and of the upper closure head of the pressure vessel, during maintenance operations which require the removal thereof.
Conventional CRDs also require the ability for obtaining relatively fast insertion of the control rods during a scram condition, which is typically provided by a high pressure accumulator which injects a high pressure driving fluid into the CRDs for quickly inserting the control rods. In the case of bottom mounted CRDs, the scram operation occurs against the force of gravity.